This invention relates to the evaluation of the fire resistance of components, in particular for air worthiness compliance of components in and around aircraft engines, although it can be applicable to fire resistance evaluations for other purposes.
Air worthiness regulations require the testing of critical components to ensure that fire in and around aircraft engines does not hazard aircraft safety. In the tests, e.g. as set out in ISO 2685, the components are subjected to flame attack from a standard flame, to evaluate an in-service fire event. An array of thermocouples are mounted on the surface of the component and the maximum temperatures recorded from the test are used to determine the viability of the component during a fire event. This can be by reference to induced thermal stresses or degradation of the material, e.g. deformation, oxidation or melting.
Testing costs are significant. At the same time, however, the accuracy of the analysis is limited by the spatial resolution of the metal temperatures that can be provided by the thermocouple array. The process is also expensive and time-consuming, particularly if a component initially fails the test and must be redesigned. This can lead to overdesign of a component due to excessive caution, with resulting cost and weight penalties.
The present invention it concerned with the development of a modelling system which allows fire resistance tests to be performed more rapidly and more economically. It is also concerned with the provision of a low-temperature heating device that can simulate the designated flame source of a standard test technique.
It is already known to model simulated high temperature conditions on aero-engine components using Perspex models with thermochromic liquid crystals on their surfaces, the models being immersed in a relative low temperature (less than 90xc2x0 C.) airstream to map the heat transfer coefficients across the surface of the model by observing colour changes in the liquid crystal. This technique (described, for example, in xe2x80x9cFull surface local heat transfer coefficient measurements in an integrally cast impingement cooling geometryxe2x80x9d, Gillespie et al (1996), ASME 96-GT-200) has been used to determine external and internal heat transfer coefficients in turbine blades.
According to one aspect of the present invention, a method is provided for evaluating the resistance of a component to a fire in which a scale model of the component is provided with a thermochromic liquid crystal on its surface and is subjected to a gas flow at an elevated temperature closer to ambient than to a hydrocarbon flame temperature, the gas density, flow rate and the scale of the model being chosen to at least substantially match the Reynolds number and Froude number of the flow over the component in the fire test conditions, and to at least substantially match the ratio of flame density to the density of the ambient surrounding in fire test conditions, and the reaction of said liquid crystal is recorded when subjected to the gas flow.
To allow the method to be performed in free air conditions, preferably the gas flow contains at least a substantial portion of a gas lighter than air. In particular, it may contain helium as a major component.
It is also desirable, to improve simulation of the flame plume of the standard flame, to ensure that the gas flow reproduces the ratio of momentum flux of the standard flame combustion products to total reactant momentum flow in the entry to the standard burner.
According to another aspect of the invention, a flame simulation device for use in low temperature simulation of a standard flame of a fire resistance test, comprising respective sources of pressure air and helium and means for producing a mixed flow therefrom in predetermined proportions and heating said flow, a duct for receiving said flow and simulating a standard test burner, said duct having an exit face formed by a plate with a series of apertures for the passage of the heated mixed gases, said apertures having an area in relation to the area of the duct downstream of the plate which produces a change of momentum flux in said flow substantially reproducing the ratio of the momentum flux of the standard flame combustion products to the momentum flux of the reactants on entry to the standard burner.